With the rapid development of digitalization of information, digitalization in image processing is increasingly used. In particular, as symbolized by digital cameras, solid-state detectors, such as Charge Coupled Devices (CCD) and Complementary Metal Oxide Semiconductor (CMOS) sensors, have been mainly provided on imaging planes instead of films.
In this manner, an image of an object is optically taken by an optical system and is extracted by a detector in a form of an electric signal in image pickup apparatuses including a lens system comprising a plurality of lenses and CCDs or CMOS sensors for detectors. Such apparatuses may be used in video cameras, digital video units, personal computers, mobile phones, Personal Digital Assistants (PDAs), image inspection apparatuses, industrial cameras used for automatic control, and the like as well as digital still cameras.
FIG. 27 is a schematic diagram illustrating a structure of an image pickup lens apparatus and traces of light beams. In FIG. 27, the image pickup lens apparatus 1 comprises: an optical system 2 comprising a plurality of lenses; and a detector 3 such as CCD, CMOS sensors and the like. In FIG. 27, the best-focus plane (BFP) of the lens system 2 coincides with an imaging plane of the detector 3. The optical system 2 comprises: lenses 21 and 22 located at an object side (OBJS); an aperture stop 23; and an imaging lens 24. The lenses 21 and 22 at the object side, the aperture stop 23 and the imaging lens 24 are arranged in order along an optical axis O-O from the object side (OBJS) toward the detector 3 side.
In the image pickup lens apparatus 1, the best-focus plane (BFP) coincides with the plane on which the detector is disposed. FIG. 28A through 28C illustrate spot images formed on a light-receiving surface of a detector 3 in the image pickup lens apparatus 1. In FIGS. 28A, 28B and 28C, the horizontal axis refers to the center of gravitational force and the vertical axis refers to depth of field. FIG. 28B illustrates a spot image formed when focused. FIGS. 28A and 28C illustrate spot images when a focal point is displaced by 0.2 mm and −0.2 mm, respectively.
For example, non-patent document 1 and 2, and patent document 1-5 suggest a pickup apparatus in which light beams are regularly blurred by a phase plate and the blurred light beam is reconstructed by digital processing to achieve a large depth of field. An automatic exposure control system for a digital camera in which filtering process using an optical transfer function (OTF) is performed is proposed in Patent document 6.
In devices that have image input functions such as CCD and CMOS sensors, sometimes, it is very useful to read close-up still images such as bar codes, as well as desired images such as landscape images. The first example of known techniques for reading bar codes is, for example, a technique in which focusing (focus) is performed in an auto-focus mode by moving a lens in an optical system. In addition, the second example is a method in which depth expansion technique is applied. The depth expansion technique, for example, is known as a technique in which the depth of field is extended by adjusting an aperture so as to reduce the f-number in a camera, thereby achieving a fixed focus.    Patent Document 1: U.S. Pat. No. 6,021,005.    Patent Document 2: U.S. Pat. No. 6,642,504.    Patent Document 3: U.S. Pat. No. 6,525,302.    Patent Document 4: U.S. Pat. No. 6,069,738.    Patent Document 5: Japanese Patent Application Laid-Open No. 2003-325794.    Patent Document 6: Japanese Patent Application Laid-Open No. 2004-153497    Non-Patent Document 1: “Wavefront Coding; jointly optimized optical and digital imaging systems”, Edward R. Dowski, Jr., Robert H. Cormack, Scott D. Sarama.    Non-Patent Document 2: “Wavefront Coding; A modern method of achieving high performance and/or low cost imaging systems”, Edward R. Dowski, Jr., Gregory E. Johnson.